1. Field of the Invention
The present invention relates multilayered filter films having wavelength selectivity and methods of making the same. In particular, the present invention relates to a multilayered filter film not having a substrate, in other words, in which the substrate for forming the multilayered film is removed, and a method for making the same.
2. Description of the Related Art
In general, multilayered filter films are produced by forming multilayered films on transparent substrates, e.g. glass substrate. However, since the optical path length increases as much as the substrate thickness in such multilayered filter films having substrates, a collecting lens is required to collect the diffused light when the multilayered filter film is provided between optical fibers as a wave combining or dividing filter, or between a laser diode and a photodiode.
A multilayered filter film not having a substrate is proposed in Japanese Laid-Open Patent No. 4-7505 in which the substrate is removed after a multilayer is formed on the substrate. In accordance with such a multilayered filter film not having a substrate, the small size in devices can be achieved corresponding to the substrate volume. Further, the collecting lens can be omitted because of the decreased thickness when the multilayered filter films are provided between optical fibers as a wave combining or dividing filter, or between a laser diode and a photodiode.
Since the multilayered filter film not having the substrate set forth above consists of only the multiple deposited films, some deformations, such as curling, occur due to the internal strain and the difference of the thermal expansion coefficient between layers, so desired optical characteristics can not be achieved. Therefore, when such a multilayered filter film is placed between optical fibers or between a laser diode and a photodiode as a wave combining or dividing filter, the tip of the optical fiber or a glass may be contacted to both surfaces of the multilayered film. However, it is remarkably difficult to handle the multilayered film since the film has a thickness of approximately 5 to 10 .mu.m.